1. Field of the Invention
The present invention is directed to projection systems, and more particularly to an apparatus for blending of images from multiple digital projectors, as are used, for example, in simulation systems with day/night scenarios.
2. Description of the Related Art
Multiple projection displays are well known in the art, having been used for many years in the film industry to create high-resolution images on large variously shaped screens (e.g. domes, cylinders and toroids, in addition to large flat screens). For example, the CINERAMA system developed in the 1950s used three separate projectors to project three images which were then combined to form a single panoramic image. More recently, video based multiple projector display systems have been used for flight simulators, wherein multiple video screens are placed next to each other to form a large tiled image display.
Because of the difficulty in ‘butting’ multiple images edge-to-edge, a significant disadvantage of such multiple projector display systems is that the images often do not appear as one single continuous image on the display screen. When multiple images are projected side-by-side and/or top-to-bottom on a single screen, a seam or overlapping region is typically created between the images. Consequently, the final display image appears either as multiple images placed side-by-side with a gap between images or, if the images are made to overlap on a single screen, with a bright line or band there between. When the images of two projectors overlap, the amount of light in the overlapped regions of the images is approximately double the amount of light observed on the screen in regions where only a single projector image resides. Therefore, the region of overlap is brighter than the balance of the image.
The above described problem applies equally to black projector output levels, and is particularly relevant to DMD type displays with a finite black level off state (unlike CRT projectors). Specifically, when displaying a black image, instead of protecting a generally uniformly black area across the entire displayed image the black image tends to brighten in the regions of overlap, giving rise to objectionable artifacts.
Attempts have been made to hide such artifacts, one such example being to electronically reduce light intensity in the regions of overlap to the same brightness levels as the regions of non-overlap. Such practices are usually implemented by adjusting the input video level to obliterate the visibility of the regions of overlap. Although electronic blending does work for CRT projectors (because the “off” state is completely black), it does not work as well for light valve LCD or DMD/DLP type micromirror projectors when generating black images, which is a matter of importance when projecting quality simulated night scenes (such as required, for example, in aircraft and ship simulators). Specifically, because the off-state (i.e. black) brightness is a minimum 000 video code value , electronic reduction of light intensity cannot occur since it is already as low as it can go (zero) so it cannot eliminate the bright region of overlap without adversely affecting the contrast ratio of the projection system. Approaches such as boosting the black level of the non-blended regions degrade contrast rather significantly.
Another prior art solution is set forth in U.S. Pat. No. 6,017,123 (Bleha et al), wherein a blending device is located in the path of light between the projection lens and the screen. The blending device smoothes off-state and on-state illumination levels in the region of overlap without reducing the contrast ratio of the projector. The blending device includes physical devices such as filters, solid masks, and/or a combination thereof as a substitute for electronic blending.
A matte box type solution has also been used wherein a hard mechanical edge slides in and out of the light path of the projector, usually in a strict linear and parallel fashion. Although this solution effectively hides the edge of a projected DMD micromirror device between the imaged area and the non-imaged DMD mask, it cannot create a convincing black blend across the entire overlap region since the density of penumbra (blurred shadow) is fixed by the projector and lens pupil geometry and distance from the lens. Consequently, the matte box type solution does not work well for both dark and light scenes and is not sufficiently controllable, nor does it work well for short throw lenses. Furthermore, it does not handle keystoned projection setups whereby the overlapped regions are not necessarily in the form of vertical or parallel lines.
Another method of creating a softer edge in the overlap region is to use optical filters, fabricated using photomask techniques or solid fabrication techniques such as stereolithography, wherein the filters incorporate comb-like or serrated edges. Typically, these masks control the blending of light by selecting the size, shape, length and density of teeth. The main disadvantages of this method are quality of the light blending, cost, size, weight and the difficulty in customizing for particular lenses, projectors, or for unique projection geometries. Mechanical light attenuators can help disguise the blend edges (similar to tape) but are not as controllable or flexible as a custom optical filter.
A further method involves placing glass plates in front of the lens, either with Neutral Density (ND) graduated filters incorporated therein, or simulated with print screen patterns of various dot density similar to the method used for printing half-screen photographs in newspapers. Various densities may be created by photographically changing dot screen density on a coarse scale, or by variable density dyes, etc. The main disadvantage of such ND filter methods is the loss of light due to back reflection on each glass or plastic surface. For seams (overlapped regions) that involve top, bottom, left and right in a dome simulator this requires a very complex single filter, or for best adjustment flexibility, multiple sheets of glass each of which affects not just the blend overlap region, but the entire image area. Such methods are also very expensive to customize, and heavy to mount.